Anode break excitation and Pflüger's law

The relations between the nodes of stimulating current and excitation were studied on frog nerve with special emphasis on anode break excitation. The threshold for anode break excitation was several times greater than cathode make threshold. Following an exceedingly strong anodal pulse, there is a period of marked reduction in anode break threshold. The data indicate that anode break excitation in whole nerve results from node membrane breakdown, as concluded in a previous investigation on the single Ranvier node. The many confusing results often obtained in demonstrating Pflüger's law on whole nerve were found to be attributable to populations of nerve fibers in whole nerve and are readily explained by the results of single-fiber experiments.

Anode break excitation on single Ranvier node of frog nerve

A quantitative study of anodal break excitation in single frog nerve fibers has been carried out. An applied anodal pulse caused an increase in the critical level of depolarization for excitation. Anode break stimulation was effective only if the anodal pulse was immediately followed by depolarization of the single Ranvier node membrane. This condition can be obtained in three ways: 1) by applying the anodal pulse during a conditioning depolarization voltage, which must be equal to or greater than the threshold value for cathodal stimulation; 2) by applying a pulse of such large amplitude so as to produce membrane breakdown, which results in depolarization by intrinsic local currents; 3) by applying the pulse to a fiber already under the influence of intrinsic depolarization resulting from deterioration or high concentrations of potassium in the external solution. True anode break stimulation can only be produced by an exceedingly strong positive pulse. The membrane breakdown resulting from such a pulse, first reported by Stampfli in 1958, is discussed and a hypothesis is presented explaining the physiological implications of this phenomena.

Effect of anodal and cathodal pulses applied during action potential at a single Ranvier node

An anodal pulse applied during the falling phase of an action potential, if weak, produces a slight enhancement of negativity of the falling phase, but if increased in amplitude produces a split of the action potential into an early and "delayed" response and finally, complete abolition of the falling phase. If the pulse amplitude is increased still more after abolition, a second response is elicited. The latency to this second response following abolition is shorter than the latency to the delayed response, and further increase of the applied pulse amplitude cannot abolish this second response. To obtain abolition of the delayed response it is necessary to apply a considerably stronger anodal pulse near the peak of the spike than later during the falling phase. The reverse is true to obtain the second response. The anodal pulse sufficient to produce anode break excitation during the action potential and elicit the second response is ineffective applied to a resting node membrane. It is postulated that: a) due to an effect of the action potential itself the membrane is being actively depolarized during the early falling phase of the spike and b) the excitability of the node membrane is actually retained both during and following an action potential in the so-called refractory period but requires "resetting" by a positive pulse in order for re-excitation to take place.